Ventilation is the physiologic process of moving a gas into and out of the lungs and thereby delivering oxygen to organs of the body and excreting carbon dioxide. During spontaneous ventilation, i.e. unassisted breathing, negative (sub-atmospheric) pressure is created within the chest and gas moves into the lungs. In spontaneous ventilation exhalation is passive.
In the practice of medicine, there is often a need to substitute mechanical ventilatory support for the spontaneous breathing of a patient. This may be necessary during respiratory failure or when patients are placed under anesthesia.
Mechanical ventilatory support may be accomplished by displacing a known volume of gas into the lungs of the patient under positive pressure (any pressure greater than atmospheric pressure). Alternatively, mechanical ventilatory support may be accomplished by creating a negative pressure around the chest cavity to mimic spontaneous inhalation. While negative pressure (sub-ambient) is occasionally used for mechanical ventilatory support, positive pressure ventilation is far more common.
Attempts have been made to provide transport ventilation devices designed to provide positive pressure ventilation. These attempts have resulted in two categories of devices; (1) minimally featured transport ventilation devices designed for use by medical personnel having limited respiratory training, these devices operating in a limited number of ventilation modes, and (2) ventilators which have a large number of features, operate in a wide range of ventilation modes and which consequently are suited for use only by medical personnel with significant respiratory training. These attempts are described in a number of issued United States patents including the following:
U.S. Pat. No. 5,211,170 discloses a portable emergency respirator containing an electrically driven air compressor for generating air flow. The air compressor can be operated in one of three different modes to produce three different types of pneumatic outputs.
U.S. Pat. No. 4,941,469 and related U.S. Patent No. 4,823,787 disclose portable ventilator units having electrically driven cyclically operated reciprocating pumps for providing pressurized air to a patient. The ventilators of these patents may be operated in a number of ventilator modes.
U.S. Pat. No. 4,905,688 discloses a pneumatically driven portable self contained ventilator/resuscitating device utilizing a solid state oxygen generator, such as a chlorate candle. The ventilator/resuscitator is designed for use by personnel with limited respiratory training and thus has limited features and ventilation modes.
U.S. Pat. No. 4,651,731 discloses a pneumatically driven portable self contained ventilator/resuscitating device utilizing a solid state oxygen generator, such as a chlorate candle. The ventilator/resuscitator has a number of adjustable features and various ventilator modes and is intended for operation by medical personnel with significant respiratory training.
These prior systems have many disadvantages. In particular, the minimally featured ventilators can maintain ventilation of seriously ill patients, however, they lack many of the advanced features found on the more sophisticated ventilators. These devices have limited utility in that they are suited only to short term ventilation such as would be necessary during transport situations.
In contrast, the more sophisticated ventilators are generally more costly, larger and require more training to operate than the minimally featured ventilators. As a result the more sophisticated ventilators are impractical for use in many environments such as aero-medical transport, in emergency departments, during intra-hospital transport and in hospitals of developing or third world countries.
In the prior devices which are microprocessor controlled or utilize electrically driven gas supplies, an electrical failure can result in an inoperative ventilator. Alternatively, the prior devices, which are pneumatically driven and controlled, lack many of the advanced safeties and features available through the use of modern microprocessor technology.
In addition, these prior devices require the initial parameters, such as tidal volume (V.sub.T), ventilatory breathing frequency (f) and inspiratory flow rate (V.sub.i) to be input by the health care provider. These values are generally determined based on the patients weight and age. In emergency situations the difficulty in accurately determining a patient's weight as well as errors in inputting the parameters can result in improper, even dangerous, ventilator settings. The prior art devices do not provide for safety mechanisms to prevent such occurrences.